Gastrointestinal stromal tumors (GISTs) are driven by KIT receptor tyrosine kinase, and most have gain-offunction mutations In KIT or occasionally in PDGFRA. Imatinib, which inhibits KIT and PDGFRA, produces a partial response or stable disease in 80% of GIST patients, but complete response is rare. Moreover, about half of the patients who benefit from imatinib treatment eventually develop drug resistance, and few other treatments are available. A common mechanism of acquired resistance is second-site KIT mutations. We propose 4 overall objectives. (1) Use genomic approaches to identify alternative signaling pathways in GIST lacking KIT or PDGFRA mutations and in imatinib-resistant GIST lacking an Identifiable mechanism of resistance. Candidate genes will be validated and pathway analysis applied to identify potential targets for therapy. (2) Identify mutations that confer resistance to imatinib or to other kinase inhibitors so as to develop guidelines for genotype-tailored therapy. (3) Investigate novel pharmacological intervention strategies in vivo in the KitV558del/+ mouse, a model of imatinib-responsive GIST. (4) Develop imatinib-resistant mouse models and apply them for the evaluation of new treatment strategies for imatinibresistant GIST. The treatment strategies to be tested are second^generation tyrosine kinase inhibitors alone and In combination with inhibitors of downstream effectors of KIT, targeting PI 3-kinase, integrin, and STAT signaling. By elucidating the pathways active in imatinib-resistant GIST and by preclinical investigations, we aim to find new therapeutic options for patients with Imatinib-resistant GIST.